The field of this invention is that of electronic thermostats for multistage heating or cooling systems and the invention relates more particularly to programmable thermostats for multistage heating and cooling systems which use heat pumps in the main stages of the systems and to methods for regulating operation of such multistage systems.
Electronic programmable digital thermostats are being used extensively and with great success to achieve substantial energy savings by providing automatic shutdown or temperature setback of conventional temperature conditioning systems during sleeping hours or when a living space is unoccupied. Many of such systems use multiple heating or cooling stages to obtain greater operating efficiencies. Some systems incorporate heat pumps to achieve other energy savings by taking advantage of the greater energy efficiency of heat pumps relative to conventional furnaces. Heat pump systems are usually of the multistage type in that they have less efficient auxiliary stages such as electrical resistance heaters which supplement the heating capacity of the heat pump means during colder weather. Frequently however, multistage systems present some comfort temperature control problems, and multistage systems using heat pump means or the like in the main system stages often fail to achieve satisfactory economies when used in temperature setback operation.
For example, under some climatic conditions, the temperature provided by a typical multistage temperature conditioning system using a conventional thermostat is permitted to fluctuate around a temperature which is offset by as much as 2.degree. F. From the intended control temperature set by the user. That occurs because, in such conventional thermostats, the temperature set by the user serves as the threshold temperature of the main heating or cooling stage of the system while the threshold temperatures of the other, auxiliary heating or cooling stages vary by up to about 2.degree. F. from the main stage threshold temperature. The auxiliary thresholds are selected and arranged so that, under most normal weather or housing conditions, the comfort temperature is regulated around the control temperature the user has selected. However, when climatic conditions are relatively severe so that the main heating or cooling stage of the system does not in itself have sufficient capacity to bring room temperature to the control temperature set by the user, the threshold temperature of a second or subsequent stage of the system can become the effective control temperature of the system. In that situation, the temperature provided by the system fluctuates around that alternate "control" temperature which is offset by as much as 2.degree. F. from the desired control temperature, thereby resulting in considerable user discomfort.
Similarly, when a temperature set back has been provided during the night, the amount of heating or cooling and the period of system operation required for recovery from the setback temperature in the morning can vary widely with changes in climatic conditions. This is particularly true where the thermal characteristics of the building are relatively poor or where the heating or cooling capacity of the system is relatively limited with respect to the size of the comfort controlled zone. As a result, where conventional thermostats control operation of the various stages of the system by reference to independent threshold temperatures for the various system stages, they frequently do not restore the desired comfort temperature at the scheduled time and either cause discomfort by tardy temperature restoration or lose potential energy savings by premature temperature restoration.
The conventional thermostats also tend to lose potential energy savings in setback operation of multistage temperature conditioning systems by the excessive or unnecessary use of the usually less efficient auxiliary stages of the system. This is particularly true in the case of heat pump systems in which very high efficiency heat pumps in the main temperature conditioning system stages are typically combined with substantially less efficient electrical resistance heaters in the auxiliary heating stages. For example, when a conventional programmable thermostat is used with a multistage heat pump system, the recovery from a setback temperature is often scheduled to occur in the morning at the coldest part of the day when a heat pump tends to operate at its lowest level of efficiency. In that circumstance, sone conventional thermostats call for immediate use of both the heat pump and the auxiliary heating means to provide quick recovery of the comfort temperature after reaching the scheduled end of the night time temperature setback period. That is, because there is a substantial difference between the setback temperature and the desired comfort temperature to be restored at the end of the setback period, both main and auxiliary heaters are used in restoring the comfort temperature and a large part of the temperature restoration tends to be effected by the less efficient auxiliary heating means. Such use of the less efficient auxiliary heating stages can result in loss of some or all of the energy savings which had been accumulated during the overnight setback period even though longer use of the more efficient heat pump means alone might have resulted in suitably prompt comfort temperature restoration in the morning in a more energy-efficient manner. Other known thermostats attempt to regulate operation of the various heating or cooling stages of multistage systems by reference to estimated heating capacities of the system components. One known system initiates system operation after temperature setback a set time before the end of the setback period whether there is a large or small difference between the setback and desired recovery temperature.
With the present emphasis on energy conservation, it is desirable to provide improved thermostats and control methods for use with a multistage heating and cooling system to obtain improved control of comfort temperatures while taking advantage of the potential energy savings resulting from the use of heat pump means and from the temperature setback type of system operation.